Reducing HIV spread among drug injectors, and from them to their sex partners, remains an urgent public health problem. Prevention will be facilitated by understanding (a) behaviors and social networks through which HIV is transmitted, and (b) how social networks and peer culture influence risk behavior. Such knowledge may also help answer key questions such as why black and Latino drug injectors are more likely to be infected than white ones; and why women drug injectors get infected earlier in their drug-injecting careers than men in some cities (but not in all---suggesting that the explanation lies at least partially in social or cultural factors); and how condom use can be encouraged. This proposal is to continue analysis of data already collected by the social Factors and Hiv Risk project. This project has already shown that social networks and peer cultures are significant predictors, under multivariate controls, of HIV serostatus and also of risk behaviors such as injecting with syringes others have used and preventive behaviors such as consistent condom use-indicating that networks and peer cultures may be important foci for intervention efforts. The project's specific aims are to use data collected on 767 drug injectors in Brooklyn, 3,165 of their dyadic (egocentric) relationships, and the sociometric relationships among 491 of these drug injectors, to: 1. Determine how the structures of drug injectors' social networks are related to HIV infection, including whether they help explain racial/ethnic differences in seroprevalence. 2. Compare drug injectors' social networks in New York with those in a low seroprevalence city (Colorado Springs) to gain insights about how HIV epidemics can be prevented. 3. Assess how social network characteristics affect risk and preventive behaviors. 4. Study how drug injectors' networks, peer cultures, behaviors, and HIV infection levels vary with years of injection, and try to determine why women and other subgroups of new drug injectors become infected earlier in their careers than do others. 5. Use mathematical modeling to determine key parameters and intervention points for reducing HIV spread. 6. Further develop and validate techniques for research on drug users' social networks, peer cultures, and social relationships so other projects can use them in future studies. This research will be used to suggest innovative techniques reduce HIV spread. Examples might be to help drug injectors avoid high-risk social network "locations;" helping those who are involved in high-risk network "cores" to change their peer cultures and behaviors; or developing interventions for new injectors, particularly those women and others whose characteristics suggest they are at particularly high risk of becoming infected.